NASA aimed for another shot Friday morning as tens of thousands of disappointed and weary launch guests hustled out. The weather report was iffy  forecasters expected higher winds, clouds and rain  but managers were optimistic the wind direction would shift in their favor.

United Launch Alliance’s Dan Collins, who’s in charge of the unmanned Delta IV rocket, said he was confident the valve trouble, seen before on a previous mission, could be quickly overcome.

“The team was absolutely on their game, listening to everything the rocket was telling us, and it ultimately told us it wasn’t ready to go today,” Collins told reporters. “And so we’ll go make sure we’ve got a happy rocket … and send Orion off to a very, very successful test flight.”

Orion is how NASA hopes to send astronauts on round trips to Mars in the decades ahead. This inaugural flight, while just 4 1/2 hours, will send the unmanned capsule 3,600 miles into space.

It’s the first attempt to send a spacecraft capable of carrying humans beyond a couple hundred miles of Earth since the Apollo moon program.

NASA’s new countdown clock  making its debut alongside Orion  got a workout as problem after problem cropped up in the final four minutes Thursday.

The initial launch was delayed by excessive wind and a cargo ship believed to be in the launch-danger zone; it later turned out the ship was in a safe spot. Then the valve trouble cropped up in the rocket’s first-stage boosters.

“It was a roller coaster: We’re going, we’re not going,” said Sarah McNulty, a space educator who was helping NASA escort the several-hundred news media on hand. “I’m running on no hours of sleep, zero, zero hours.”

An estimated 27,000 guests gathered for the historic send-off  roads leading into Kennedy Space Center were packed well before dawn  and the atmosphere was reminiscent of the shuttle-flying days. “Go Orion!!” urged a hotel billboard in nearby Cocoa Beach.

Astronauts in attendance included Anna Fischer, one of NASA’s original spacewomen, and now assigned to the Orion program. She too was disappointed by the delay, but said “It’s way more important to have a successful flight.”

“It was so much fun to come out here and have that same atmosphere as before a shuttle launch,” Fisher said. “We really miss that. That’s why we’re here. This is what we love.”

Orion is aiming for two orbits on this inaugural run, reaching a peak altitude of 3,600 miles, high enough to ensure a 20,000 mph re-entry at a scorching 4,000 degrees. Splashdown will be in the Pacific off the Mexican Baja coast, where Navy ships are waiting.

The $370 million test flight  excluding the price of the capsule, which will be reused in a launch abort test  covers all delay-associated costs for the near term, according to officials.

The spacecraft is rigged with 1,200 sensors to gauge everything from heat to vibration to radiation. At 11 feet tall with a 16.5-foot base, Orion is bigger than the old-time Apollo capsules and, obviously, more advanced.

NASA deliberately kept astronauts off this first Orion.

Managers want to test the riskiest parts of the spacecraft  the heat shield, parachutes, various jettisoning components  before committing to a crew. The earliest Orion might carry passengers is 2021; asteroids are on the space agency’s radar sometime in the 2020s and Mars, the grand prize, in the 2030s.

Lockheed Martin Corp., which is handling the test flight for NASA, opted for the powerful Delta IV rocket this time around. Future Orion missions will rely on NASA’s still-in-development megarocket known as SLS, or Space Launch System. The first Orion-SLS combo launch is targeted for 2018.

NASA’s last trip beyond low-Earth orbit in a vessel built for people was Apollo 17 in December 1972, the last time men set foot on the moon.

“It’s a thrilling prospect when you think about actually exploring the solar system,” space station commander Butch Wilmore said from orbit before the delay. “Who knows where it will take us?”